Climate Change (Part 1 of 2)

You do not have Javascript enabled. Some elements of this website may not work correctly.

Notice about research

Giving What We Can no longer conducts its own research into charities and cause areas. Instead, we're relying on the work of organizations including J-PAL, GiveWell, and the Open Philanthropy Project, which are in a better position to provide more comprehensive research coverage.

These research reports represent our thinking as of late 2016, and much of the information will be relevant for making decisions about how to donate as effectively as possible. However we are not updating them and the information may therefore be out of date.

1. Importance

Anthropogenic climate change poses a significant threat to human well being on a global scale. It has been claimed by leading health journals that dealing with expected warming “…could be the greatest global health opportunity of the 21st century…”.[1] The Earth’s mean surface temperature has already increased by 0.8°C since the beginning of the 20th century,[2] and 0.6°C since only 1980.[3]

This increase in temperature has been attributed primarily to increased concentrations of greenhouse gases in the Earth’s atmosphere, which is itself attributed to the release of such gases through human activities such as energy production, agriculture, transport and land clearing.[4] For instance, the concentration of carbon dioxide, one of the most prevalent such gases, has risen by more than 40% since the start of the Industrial Revolution, from approximately 280 parts per million (ppm) to over 400 ppm (see Figure 1).[5] Notably, prior to 1950, the highest concentration over the last 400,000 years is estimated to be roughly 100 ppm less than our current level.[6]

Figure 1 (from the National Oceanic and Atmospheric Administration, United States): Proxy measurements of atmospheric concentration of carbon dioxide over time, taken from ice cores.[5:1]

It has been confirmed by the International Panel on Climate Change that the release of greenhouse gases due to human activities is the primary driver of the recent temperature increase, with 95% certainty.[7] Indeed, despite considerable uncertainties in the exact relationship, modelling has indicated that every metric tonne of CO2 or CO2-equivalent which is emitted by humans results in a roughly linear increase in global atmospheric temperature by approximately 0.0000000000015 degrees Celsius.[8] Based on projected business-as-usual emissions scenarios, if emissions from human activities are not significantly reduced in future, this will result in average surface temperature increasing by approximately 3.6°C by 2100 (with a 90% confidence interval of 2.3-5°C).[9],[10]

1.1. Extreme Events

The impacts of such a temperature increase would be extensive. For instance, a variety of extreme weather events would likely increase in both severity and frequency,[15] as has already been observed for periods of extreme heat, heavy precipitation events, fluvial floods and tropical cyclones.[16] Increased climate variability, also attributed to temperature increases, has already contributed to increased human mortality and greater disruptions both to ecosystems and to human wellbeing through increased incidence of heat waves, droughts, extreme precipitation, inland and coastal flooding, storm surges, landslides, wildfires, and cyclones.[17] This trend will only continue: under the high-emissions, high-growth scenario RCP8.5 (‘Representative Concentration Pathway’ 8.5), the incidence of heat waves and droughts is expected to reach the point of compromising essential human activities such as growing food and working outdoors in many areas.[18] In addition, the strongest cyclones in the Atlantic are expected to double in frequency by 2100[19] - as with the increased incidence of other extreme events, this is likely to be accompanied by an increased death toll if there is not sufficient adaptation in advance.[20]

From the 0.8°C warming thus far, the increased likelihood of droughts and other such events may have already been felt – the current drought in Ethiopia has already left approximately 425,000 children in need of treatment for severe malnourishment[21] and is expected to leave 15 million in need of food aid.[22] The 2003 European heat wave resulted in 70,000 excess deaths.[23] The 2010 floods in Pakistan, which led to over 2,000 deaths and affected 18 million people in total, may be at least partially attributed to the changing Indian monsoon season.[24][25] The 2007-2010 drought in Syria, made 3 times as likely by climate change, led to the migration of up to 1.5 million people from rural to urban areas, and has since been identified as a significant contributor to the loss of life in the country’s recent civil war (although it has been argued elsewhere that resource shortage does not correlate well with war).[26][27]

Apart from exacerbating armed conflicts, such droughts and other extreme events also impact heavily and disproportionately on the poor, with weather events in India cited as a leading cause of families returning to poverty.[28] The World Bank estimates that, by 2030, climate change will have pushed 100 million people back into poverty.[29] Between 2030 and 2050, it is also estimated that a business-as-usual scenario would result in an additional 720 million people being put at risk of extreme poverty,[30] which is as many as have exited extreme poverty in the past two decades.[31] These effects are also expected to increase further with additional temperature increases.[32]

1.2. Food Supply

In addition to extreme weather events, climate change is also expected to adversely affect the food supplies of many of the world’s poor. Due to increased temperature and reductions in renewable surface water and groundwater, the production of staple crops such as wheat, rice and maize is expected to be adversely affected.[33] With 2°C of warming, crop yields in Africa are expected to be lowered by 5%.[34] At 2.5°C of warming, this would reach 15%.[35] With warming of around 4°C, global food security would be at threat.[36] In Sub-Saharan Africa in particular, where up to 60% of household income is spent on food, average food prices are expected to rise 12% by as early as 2030.[37] Worldwide, a meta-analysis of 1,700 different crop yield simulations has shown that there is majority consensus that crop yields will be negatively affected from 2030 onwards and that decreases are projected to exceed 10% in the latter half of the century.[38] This is made somewhat worse by the result that all of the yield changes projected to be positive will occur in temperate regions which are generally wealthier, so the impact on the vast majority of the world’s poor located in equatorial regions will be even greater.[39] These decreases in crop yields and the resultant food shortages are expected to increase undernutrition, micronutrient deficiencies, and stunting in the developing world.[40],[41] In particular, by 2050 there will be an additional 25 million children who are undernourished and, by 2030, incidence of severe stunting in Sub-Saharan Africa and south Asia and might increase by 23-62% due to climate change.[42]

In addition to reduced crop yields, it is expected that changes in climate will impact heavily on marine biodiversity and the global distribution of marine species.[43] It is also expected that this greatly reduce the productivity of fisheries and contribute further to food shortages in sensitive areas.[44]

1.3. Water Supply

Water shortages are another expected impact of climate change. With the continued reduction of snowpacks due to warming, a large portion of the 2 billion people who currently rely on melting snow for their water supply will be affected.[45] With 2°C of warming, 400 million of these people will be put at risk of water scarcity.[46] Flooding and drought will also affect the availability of safe drinking water in many areas, which will impact heavily on water quality and hygiene.[47] One effect, which has already been observed in Bangladesh, is the increase in maternal and pre-natal deaths due to high blood pressure during pregnancy as a result of increased salt content of drinking water.[48] Another effect will be an increase in diarrhoeal illness, with 48,000 additional children expected to die of such illnesses each year by 2030.[49] An increase in temperature will also prevent the use of highly effective low-cost methods of water disinfection such as solar disinfection, which more than 2 million people rely on, from being utilised in many areas where daytime temperatures exceed 45°C.[50][51]

1.4. Disease

Of particular concern, temperature increases are expected to greatly increase morbidity and mortality due to disease - malaria, dengue fever, and schistosomiasis, among others, as well as undernutrition and other non-communicable conditions mentioned above.[52][53][54][55]

Temperature increases are expected to greatly increase the area suitable for malaria transmission and the population at risk. For instance, malaria incidence in Nepal has already increased by 25% with only 1°C of warming[56] and, worldwide, 2-3°C of warming is expected to put an additional 150 million at risk for malaria[57] - this is equivalent to approximately a 5% increase worldwide.[58] As a result of this, the WHO estimates that warming under a medium-high emissions scenario will result in 60,000 additional malaria deaths each year by 2030 (dropping to 33,000 additional deaths by 2050, due to global health initiatives).[59]

Cases of dengue fever are also set to increase considerably (with the potential for epidemics increasing by 31-47% in areas studied).[60] Cases in Nepal have already been observed to rise significantly due to climate change.[61] In Sri Lanka, dengue cases increased by 600% between 2008 and 2014, from only 6,600 infections each year to 47,000, with a corresponding increase in fatalities.[62] Globally, the WHO estimates that warming under a medium-high emissions scenario will result in only 258 additional deaths each year (climbing to 282 by 2050),[63] however the impacts of dengue fever are made up more by severe illness and morbidity than by mortality (with dengue making up 3 times more of global DALYs than of global deaths).[64][65]

Likewise, the prevalence of the parasitic worm infection schistosomiasis is expected to increase in some areas due to more areas becoming suitable habitats for snails - the schistosomiasis vector. In eastern Africa, it has been estimated from computational models that schistosomiasis prevalence will increase by up to 20% over the next 20-50 years due to climate change, with particularly large increases experienced in Rwanda, Burundi, Kenya and Zambia (all of which, apart from Kenya, are areas where SCI operates).[66][67] Such a 20% increase equates to roughly 242,000 additional DALYs every year[68] and threatens to undo much of the work done to reduce schistosomiasisprevalence.

1.5. Total Impact

Aggregating only a few of the many adverse effects of climate change, the World Health Organisation estimates that deaths due to increased temperatures reached 150,000 per year in 2000.[69] Even assuming continued progress on health, it is estimated that each year from 2030 to 2050 will see approximately 38,000 additional deaths due to heat exposure, 48,000 due to diarrhoea, 60,000 due to malaria, and 95,000 due to childhood undernutrition.[70] Excluding the numerous other diseases, extreme weather events, and unrelated factors which are expected to contribute heavily to human mortality and morbidity, these four factors are thus expected to lead to 250,000 deaths per year from 2030 to 2050[71] and, without considerable adaptation and further advances, this can be expected to continue indefinitely beyond 2050. This is likely to be a considerable underestimate of the effect of climate change on total mortality, due to the many other factors not considered although these causes of death would likely constitute a large portion of the total death toll. 250,000 could therefore be considered a lower bound on the number of additional deaths each year due to climate change. In our post on modelling the cost-effectiveness of climate change mitigation, we estimate that 4.8 million additional deaths per year is a very high upper bound. This is calculated by supposing that all causes of death - cancer, dementia, mental illness, road traffic injuries, and so on - all increase by the same proportion as the those considered above. Even supposing that deaths due to natural disasters (0.035% of deaths globally) and armed conflict (0.057%) would increase by a much greater amount, they would need to increase by more than 3 orders of magnitude to counterbalance the overwhelming majority of causes of death which are unlikely to increase by the amount estimated (or, indeed, at all). Therefore, the true figure for additional deaths per year due to climate change between 2030 and 2050 is very likely to be greater than 250,000 but a great deal less than 4.8 million.[72]

The figure of 0.25-4.8 million deaths per year also does not account for low-probability catastrophic outcomes (so-called ‘tail risks’) which may constitute a sizeable portion of the expected harms produced by climate change, although there is limited information available about the impacts that could be expected in these scenarios. In addition, the figure of 250,000 human deaths does not account for the impacts of climate change in areas other than human health, such as on animal welfare, biodiversity and the state of the natural environment. There is less evidence and understanding of the impacts in these areas, and fewer metrics available to evaluate them, but it is expected that they will beconsiderable.

1.6. Tail risks

Even though unlikely, there is a small probability that the effects of climate change may be considerably worse than 250,000 deaths per year. For warming by 2100 in a business-as-usual scenario, the IPCC has established a 90% confidence interval from 2.3°C to 5°C.[73][74] That is, there is a 5% probability that warming will be less than 2.3°C, and hence far less warming than the median estimate of 3.6°C and likely far fewer adverse impacts on human health, biodiversity and the natural environment. However, it also means that there is a 5% probability that warming will exceed 5°C which, given the generally positive relationship between warming and adverse impacts, can be reliably expected to result in far more human deaths and far greater impacts on biodiversity, the natural environment, economic productivity and various otherareas.

Five degrees may be lower than the IPCC’s previous 95th percentile estimates, which were as high as 6.4°C in 2007.[75] However, warming of 5°C above pre-industrial levels could still bring about considerably large impacts. Although there is minimal research available on the impacts of warming at such levels, a detailed risk assessment by the Centre for Science and Policy at the University of Cambridge found that such levels of warming could have far greater impacts on heat stress, crop yields, water availability, drought, flooding and sea level rise than is estimated at median levels of warming.[76]

At 5°C, the probability of potentially lethal daytime temperatures - at which hyperthermia and tissue damage would occur within several hours of exposure - during the hottest month of the year in large areas of China and the United States is estimated to rise to almost 20%.[77] In Northern India, it climbs to roughly 30% (see Figure 4 below).[78] This would render large portions of these countries uninhabitable for part of the year, or otherwise vastly increase the annual death toll. But even without reaching lethal temperatures, the probability of reaching temperatures at which humans are physically unable to perform medium/heavy labour rises above 20% in areas of China and the United States, and to over 60% in North-Eastern India.[79]

Figure 4 (from Centre for Science and Policy): Probability (%) that a person in a region is exposed to heat that causes core body temperature to rise to 42°C, for an average individual at rest in the shade for 4 hours. Defined as ‘Wet Bulb Globe Temperature’ greater than or equal to 40°C, for 10% of the days in the hottest calendar month of the year. Temperature increase is relative to present day.[80]

Likewise, at 5°C of warming, crop failure due to high temperatures is expected to be widespread, particularly for maize and rice. The probability of exceeding the crop’s temperature threshold at least once during flowering increases, for instance, from 0 at present to 25-80% for rice in Eastern China, and from 0 to 6-40% for maize in Northern regions of the United States (note that these projections are at only 4.7°C of warming).[81] Similar increases are also expected for wheat, maize and rice crops elsewhere in the world. This is concerning as it far exceeds the food supply issues expected for 2°C or 2.5°C of warming - reductions in crop yields in the most vulnerable areas of roughly 5%[82] and 15%,[83] respectively. Instead 5°C of warming can be expected to lead to complete crop failures even in the temperate regions from which much of the developed world source their food, of 80% for some forms of rice and 40% for some forms of maize. This indicates a problem of far greater scale than is likely at lower levels ofwarming.

In addition, 5°C of warming would result in the incidence of extreme drought in Southern Asia and the United States increasing by roughly 50%.[84] The amount of cropland affected by drought globally would also more than double,[85] resulting in even greater risks to food security as well as water shortages. At the same time, while a scenario with warming of 2°C would result in roughly 30 million additional people being affected by river flooding each year, a 5°C scenario would result in roughly 110 million additional people being affected by flooding.[86] As with lethal daytime temperatures and crop failures, the increased incidence of drought and flooding at 5°C represents an enormous increase over lower business-as-usual estimates. With impacts of this magnitude, even a 5% probability of a business as usual scenario resulting in this outcome would increase the expected impacts of climate change considerably, as well as the expected value of preventing or minimising climatechange.

If we consider the future beyond 2100,[87] the situation becomes worse still. The 90th percentile of business as usual emissions results in a 50% probability of exceeding 7°C of warming before 2200.[88] There is even less research into the potential impacts of a 7 degree temperature change, but conventional modelling has indicated that the economic cost of 6°C would be roughly 4 times greater than the cost of 2.5°C.[89] The risk assessment by the Centre for Science and Policy also indicates that heat stress, crop yields, drought and flooding all become much worse even than in 5°C scenarios. The probability of lethal temperatures jumps to more than 60% in areas of the United States and China and to more than 90% in Northern India; the number of additional people affected by river flooding increases to 200 million; and the amount of cropland affected by drought worldwide would increase by more than 200%.[90] However, it is conceivable that humanity might be able to adapt to or reverse the effects of climate change in the future due to improved technology (see Tractability - Geoengineeringbelow).

The possibility of these risks, and the uncertainty over whether they may or may not occur, is largely a result of uncertainty over the exact temperature increases predicted in a business-as-usual scenario. However, this uncertainty may be even greater in the calculation of impacts, due to the shortcomings of models and the extreme difficulty of estimating impacts in so many different areas of human life in a world that will likely be very different from the world we currently inhabit.[91][92] It has been recognised that there are “…deep uncertainties about virtually every aspect of the natural and social sciences of climate change…”,[93] as well as that the models which are widely relied on to assess impacts - integrated assessment models (IAMs) such as ‘DICE’ - may be extremely inaccurate, particularly for assessing catastrophic events.[94][95][96][97] The IAMs, on which all of the above estimates rely, are relatively simplistic and omit a wide variety of important factors.[d][98] Due to this, they vastly underestimate low-probability catastrophic outcomes.[99] Thus, not only is there a great deal of uncertainty in assessments of the impacts of climate change - which would be enough to greatly increase the risk of catastrophic impacts - but the models on which those assessments rest are known to neglect many types of impact and the extent of potential catastrophes. Given this, it is likely that the impacts of 5°C and 7°C warming would actually be a great deal worse than were describedabove.

The probability and magnitude of catastrophic warming is increased even further by the presence of recognised feedback loops, perhaps the most well-known of which is the thawing of arctic permafrost. It is estimated that 500Gt of carbon are frozen in the Yedoma region of Siberia and Alaska, predominantly in the form of methane, and 900Gt worldwide (in comparison, anthropogenic emissions in 2016 are expected to be less than 15Gt).[100][101] Were this carbon released, it would double the atmospheric concentration,[102] thereby increasing temperatures far beyond the 7°C level. With the Arctic warming roughly twice as quickly as the global average,[103] it is likely that current projected warming will result in widespread thawing of permafrost and the release of some portion of those 900Gt.[104] Current projections estimate that a median business-as-usual scenario (3.6°C temperature rise) would likely only experience roughly 0.17°C of additional warming in 2100 (and 0.29°C in 2200) due to feedback loops from thawing of the permafrost, but these projections are still quite uncertain and there is some probability of a much greater amount of carbon being released from permafrost, which contributes further to the risk of catastrophicwarming.

In a business as usual scenario, there is evidence that the probability of “…a disastrous collapse of planetary welfare is non-negligible..”.[105] There is limited research into the small probabilities of catastrophic climate change but these might still be approximated based on different possible shapes of the probability distribution of climate sensitivity - whether it is a Pareto/’fat-tailed’ distribution, a normal/’thin-tailed’ distribution, or a Lognormal distribution which is partway between the two.[106] Based on these distributions, all of which are plausible, the probability that a forecasted 3.6°C[107] of warming (the current median business as usual estimate of the IPCC for 2100) actually results in more than 10°C of warming may be 1.4%, 0.1% or 0.0007%.[108] In addition, the probability of a more than 20°C rise in average global temperature may be 0.2%, 0.0006% or 0.00000000000000000000000000004%.[109] Given the extreme uncertainty in estimating probabilities of these events, and hence the uncertainty of which of these distributions and probabilities are accurate, it is entirely plausible that the fat-tailed distribution is accurate and that there is a 1.4% probability of exceeding 10°C and 0.2% probability of exceeding 20°C, which are indeed non-negligible. On another level, it is also plausible that there is also a non-negligible probability of humans burning all of the available fossil fuels, which is estimated to lead to 16°C of warming, even at median rates of climate forcing.[110] In reality, we have very little understanding of what the probability distribution might actually look like so this is all highly uncertain, but it certainly appears that the probability of extremely high levels of warming is not at all negligible. If the impacts of 10, 16 or 20°C temperature increases are sufficiently large, this may then mean that mitigating climate change is far more important thansupposed.

Again, the actual impacts of large temperature changes are highly uncertain. Nonetheless, there is evidence that these impacts would be enormous. For instance, average temperatures 10°C above current levels were last experienced roughly 55-34 million years ago, and accompanied by the complete absence of ice and tropical conditions at the poles.[111] In addition, from the risk assessment discussed above, it is also estimated that warming of 11-12°C would result in lethal air temperatures occurring at least once a year in areas where more than half of the world population currently lives.[112] In the areas of the United States, China and India which were studied in greater detail, the probability of such lethal temperatures occurring for 10% of days in the hottest month of the year would be close to 100%.[113] This would render much of the Earth’s surface uninhabitable for much of the year and manual labour in many areas infeasible, as well as exacerbating all of the impacts described in previous sections, threatening human security, and even posing an extinction risk for humanity.[114][115]

A 1%, or even 0.1%, chance of making much of the Earth uninhabitable does establish climate change as a cause area of high importance, with or without the more likely impacts of median business as usual scenarios. However, this does not imply that it is also an area which is also sufficiently tractable or neglected, such that donors can hope to have a large impact (see below). Whether one considers it a worthwhile area to pursue will also depend on one’s moral view and attitude towards low-probability catastrophes and existential risks - in particular, if one is averse to the risk of one’s donations having no actual effect, then attempting to reduce a 1% probability to a 0% probability may not be appealing. However, even for donors who do want to mitigate existential risk, climate change may not be the greatest risk in expectation nor the risk which might be most tractable to work on. For instance, pandemics, nuclear war, unstable political institutions and dangerous new technologies may all pose greater threats and may also be more tractable to address. Giving What We Can has not prioritised research into these areas, but we recommend the book Global Catastrophic Risks[116]as an introduction to the area and the research of the Open Philanthropy Project for donors who are interested to find out more (see also the 80,000 Hours report Climate Change (Extreme Risks)).

1.7. Other impacts

The importance of addressing climate change may also depend largely on factors other than human welfare and the possibility of human extinction. Such factors may include: biodiversity; animal welfare; the preservation of the natural environment; the survival of ecosystems; and various others. The moral relevance of these may, however, vary considerably according to individual moral views and there are numerous empirical complexities, so we have not yet been able research this areacomprehensively.

The IPCC predicts with high confidence that a large portion of animal and plant species do face increased risk of extinction this century due to climate change.[117] Even in low-emission scenarios where warming by 2100 is slightly less than 2°C, more than 50% of plant species will be unable to naturally spread to new geographical areas quickly enough to keep up with projected changes in climate, nor even will most small mammals and freshwater snails- in the median business as usual scenario of 3.6°C and those lower-probability scenarios of 5°C or more, it is even less likely that those species will survive.[118]

Marine organisms, in particular, will be affected particularly badly - not only will ocean temperatures increase and sea-ice reduce, but it is also expected that oxygen levels will drop and ocean acidification will increase.[119] The ecosystems located on coral reefs and in polar regions are expected to be even more vulnerable than those in other areas.[120]

Specifically, it is projected that business as usual will result in 57% of plants and 34% of animals losing at least 50% of their present climatic range by the 2080s.[121] Early estimates have suggested that, in sampled regions (20% of the land area of the Earth), 15-37% of all land-based animal and plant species would be “committed to extinction” by 2050 - although this percentage increases with temperature.[122][123] 15-37% equates to 0.75-1.85 million species, 8,700-21,400 of which would be vertebrates.[124] However, effective mitigation of climate change may reduce species loss by roughly 40-60%.[125]

These estimates, however, are all made with a large degree of uncertainty and some are quite outdated. Nonetheless, it can be expected that unmitigated climate change will have considerable impacts on animal and plant species, and hence also on ecosystems, biodiversity, animal welfare and the natural environment. For donors who place considerable value on these, climate change mitigation may indeed be a cause area of high importance, although there is not sufficient evidence to reliably predict the exact impact of individual donations on preventing the extinction of various species or protectinghabitat.

2. Neglectedness

Climate change is an area which appears to be far from neglected - in 2015, global investment in responding to the threat of climate change reached US$391 billion (see figure 5 for a breakdown of the spending).[126]

Figure 5: Global spending on projects related to climate change, including mitigation as well as funds intended for climate-related development aid and other purposes.[127]

This exceeds the annual investment directed towards cancer research (at less than $50 billion per year),[128]HIV research ($1.25 billion per year),[129] malaria control in sub-Saharan Africa ($1.3 billion per year),[130] and any of the other interventions which we have previously examined, which might initially suggest that the area is far from neglected. Assuming that a reasonably large proportion of that $391 billion is spent cost-effectively, or that even 0.5% is spent on the most high-impact responses to climate change, this indicates that it might be unlikely that additional donations will have a sizeable marginalimpact.

However, as a factor affecting the impact of donations, neglectedness is not a necessary condition for donations to have a high impact at a low cost but, in some cases, a heuristic which may or may not be useful. If current investments are made without regard for effectiveness, or if there is sufficient scale and tractability, there may still be opportunities for cost-effective and high-impact charitable interventions. For instance, globally, $6.5 trillion is spent each year on health,[131] approximately $1 trillion of it in non-OECD countries, and yet there is sufficient inefficiency that interventions remain which are both inexpensive and highly effective (see our top recommended charities for examples). It is possible that climate change is similar, if the scale of the problem still exceeds the $391 billion of funding which it receives each year and if the interventions which remain are sufficientlycost-effective.

For instance, spending on research and development of renewable energy technologies is still considered extremely underfunded. To spur greater renewable uptake, the optimal ratio of funding for deployment subsidies to R&D is approximately 35 to 65 for wind power and 66 to 34 for solar.[132][133] In the EU, however, the actual ratio remains at roughly 99.99 to 0.01 (and the situation is similar across the OECD).[134] Given this, R&D of renewables remains a relatively neglected area, and one which would be a prime target for political advocacy and lobbying (although we have not yet identified any highly effective advocacy organisations working on thisissue).

2.1. See Part 2 of this report for our analysis of tractability of climate change interventions and the charities working onthem.

It is worth noting, however, that estimates such as this all rely on Integrated Assessment Models (IAMs) and other models which have been criticised for being somewhat unreliable, and for potentially underestimating the risk of catastrophic outcomes. ↩︎

This is seldom done in the literature, so information about impacts beyond 2100 is more limited. This means that predictions are more uncertain, in addition to the uncertainty inherent in predicting events that far into the future. ↩︎

In Weitzman’s paper, he examines the probability of climate sensitivity exceeding these levels, rather than projected warming by 2100. Given that his median climate sensitivity is 3°C, however, his results still roughly approximate warming in 2100 under business as usual. Indeed, they provide slightly low estimates of probability. ↩︎